Voltage controlled oscillators (VCOs) are used within phase-locked loops (PLL) for clock synthesis, bit synchronization, etc. (refer to "Phase-Locked Loops", by R. Best, for background descriptions of operation and applications of PLLs). Voltage controlled ring oscillators tend to have a nonlinear transfer characteristic. This is due to the inherently nonlinear response of the current controlled oscillator which is a subcircuit of the VCO. Most designs assume linearity, which can be a reasonable approximation for small increments. However, prior techniques for implementing PLLs generally do not exhibit such linearity across a wide frequency range of operation.
A typical phase-locked loop is shown at 50 in FIG. 10. The input reference phase 52 is compared at 58 to the output 54 of the VCO 62. The charge pump 59 regulates the amount of charge on the loop filter 60. The output 61 of the loop filter 60 is a voltage that controls the frequency of the oscillator 62. A VCO 62 typically consists of two stages: a voltage-to-current (V-I) converter, and a current controlled oscillator (ICO). The VCO output frequency varies proportionally to the input voltage of the V-I converter.
For the design of PLLs, it is desirable to have a linear response across the entire frequency range of operation. This helps maintain stability and tracking in the PLL. U.S. Pat. No. 5,477,198, entitled "Extendible-Range Voltage Controlled Oscillator", and which is hereby incorporated herein by reference, teaches a technique for extending and linearizing the transfer characteristic of a voltage controlled oscillator. While feasible, this technique requires a booster inverter biasing scheme.